Coating on metals



Patented Jan. 13, 1948 COATING ON METALS Robert M. Thomas, Charles W. Ostrander, and Walter H. Kaelin, Baltimore, Md., assignors, by mesne assignments, to Rheem Manufacturing cisco, Calif., a corporation Company, San Fran of California No Drawing. Application August 23, 1943,

- Serial No. 499,710

1 Claim.

This invention relates to the treatment of zinc and cadmium and castings thereof and particularly galvanized metal, but may be used in the treatment of copper or brass and other copper alloys, including bronzes.

The primary object of the invention is to form on the metal surface an insoluble, integral, corrosion-resistant coating which is visible and may be produced in a variety of shades.

Many dip compositions employing chromium for coating metals are known and commercially available. The present invention is an improvement in such dips and possesses the characteristics of reliability and simplicity, and may be carried out at commercially acceptable costs.

We have discovered that Whereas soluble chromium compounds alone do not produce a desirable coating, a very effective coating is produced when certain anions are added to the solution. Among these are soluble compounds containing the following anions: Br-, SON, S203- 103*. We use chromic acid, or dichromates, such as sodium or potassium dichromate, together with such soluble salts as potassium or sodium iodate, bromide, thiocyanate, and thiosulfate.

The specific anions mentioned above, such as iodate, bromide, thiocyanate, and thiosulfate, have the special virtue of promoting the oxidation-reduction reactions that occur between the metal surface to be treated and the chromium compound. More specifically, they also permit the formation of a chromium compound with the base metal which is formed as an integral part of the metal surface and thus resists rubbing off or flaking and chipping. This formation of an integral surface compound goes beyond the ordinary meaning of adhesion, which implies the gluing or sticking together of two dissimilar ma terials by attractive forces inherent in one or both materials. It is to promote the formation of this integral surface compound of the base metal and the chromium compounds that necessitates the use of anions of the order of those mentioned above, in preference to other anions which conceivably may result in the formation of a reduced chromium compound with some tendency to adhere to the base metal but which can be rubbed off to a greater or less extent,

By varying the concentration of the ingredients of the bath or dip solution, and also the temperature of the solution and period of treatment, the composition of the formed coating, may be controlled within limits to produce a variety of colors, ranging from a golden yellow through orange, to greens and black, colors which reflect the greater or lesser extent to which the chromium compounds have been reduced from a valence of 6 to a lower valence. Precise shades of color .are considered to be determined by a mixture of chrome ions of several valences, in combination with the base metal, and to the presence of compounds of chromium, such as chromic dichromate, either alone or in combination with other ions.

The clip composition reacts with the metal surface and for purposes of illustration, in the case of galvanized metal, either hot dipped or elec-. troplated, forms the coating on the surface of the relatively thin surface layer of spelter metal or the pure zinc plated layer to produce the visible corrosion-resistant coating.

In order that the invention may be more clearly understood, we will describe several satisfactory examples or aqueous dips.

gms./liter 1. Sodium bichromate 13o Potassium bromide 27 Sunamlc arm 25 2. Sodium bicnromate 83 Chrome acid 33 Potassium bromide 27 3. Chromlc acid Potassium bromide 27 4. Chromic acid 20 Potassium iooate 10 5. Cnromic acid 100 'lnlocyanate (sodium or potassium) 27 6. Cnrormc acid 500mm thiosulfate l5 Hydrobromic acid, and ammonium, sodium, and zinc bromides may be substituted in th foregoing formulations for the bromide constituent.

In the case of formulations 1, 2, and 3, the period of immersion of the metal to be coated is between about 5 to 120 seconds at a temperature of about 60 F. to 212 F., whereby a dark-to-light bronze coating is produced.

In the case of formulation No. 4, the immersion time is the same as described above, and the temperature is preferably maintained at about 180 F. to 212 F. for obtaining a bronze coating.

In the case of formulations 5 and 6, the time I periods are as recited above for producing a bronze coating in the case of Examples 1, 2, and 3, and this is likewise true of the temperature ranges.

Instead of an immersion treatment, the coating may be accomplished by subjecting the surface to be coated, to suitable spraying with the solution.

After th coating has been accomplished, the articles are rinsed in hot water and suitably dried as in an oven or by blowing hot or cold air over the same, whereupon they are immediately ready for use.

By treatment of the metal surface with solutions as above set forth, the rapid and destruc tive formation of zinc salts, such as oxides and hydroxides that form in the presence of moisture or moist air, are prevented;

The treatments just above described are used equally well for treating copper, brass and other copper alloys, including bronzes.

In referring in the claim to zinc or cadmium, we intend to include pure zinc and pure cadmium, as well as mixtures thereof, articles coated with zinc or cadmium, or mixtures thereof, so-called.

zinc and cadmium castings and, of course, electroplated and hot dipped galvanized metal.

In referring in the claim to copper, we intend to include copper and its alloys, of which brass is the most prominent example, as well as bronze alloys.

We claim:

The method of" producing an integral corrosion-resistant coating on zinc, cadmium or copper comprising subjecting the same to an aqueous dip essentially consisting of water, a water soluble chromium compound selected from the group consisting of chromic. acid and salts thereof and. an alkali metal}- thiosulfatein smaller amount than said chromium compound. ROBERT M. THOMAS. CHARLES W. OSTRANDER. WALTER H. KAELIN.

REFERENCES CITED The-following references are of record in the file of this patent: 

